The T-Boom Yagi.

 

 


 


In late 2008, I started to think about building a new antenna for 1296 MHz.  At the time, I was using a single 39-element DL6WU-style Yagi, constructed to dimensions calculated using the Excel spreadsheet created by David, VK3AUU, later modified by Peter, VK3PF.  It used 4.74 mm aluminium tube for all the passive elements, mounted through a 12.5 mm box-section aluminium boom.  The driven element – a folded dipole - was made from 3.2 mm copper tubing.

 

Of course, the main driver was for more gain, which meant either a longer boom, with more elements, or multiple antennas in an array.  Or both.  Naturally, I chose both…

 

I also wanted to use a different element thickness, as the 4.74 mm tubing wasn’t readily available any more.  I’d also paid attention to people suggesting that using 4.74 (or 6.35) mm tubing at 1296 MHz was akin to using 50 mm tubing for elements at 144 MHz.  I really needed to go thinner.

 

One person I discussed this with was Gordon, VK3EJ (formally VK2ZAB).  Gordon mentioned an idea he had had a few years earlier, but had never acted upon: that of using “Tee” profile aluminium as boom material.  “Tee” profile looks, in cross section, like the letter T.  The idea was to turn the material upside down, and mount the elements through the top of the upright portion of the T (within a millimetre of the top).  With the element in its final position, the top of the boom would be punched down using a cold chisel, holding the element firmly in place – full metal-to-metal contact.

 

The “Tee” profile chosen for this project has a wall thickness of 3 mm, and the “upright” and “base-plate” portions of the T measure 25 mm (refer Diagram 1: Tee Profile).  In Australia, it is available in lengths up to 6.5 m long from Capral, and should be available elsewhere in shorter (2 m) lengths.  The Capral Product Code for this profile is currently 841506, though it used to be called “E20193MF605400” in the old Capral “Little Blue Book”.

Text Box: Upright
 

 

 

 

 

 

 

 

 

 

 


To kick things off, I built two 10-element Yagi; one scaled for 1296.200 MHz, and another scaled for 1090 MHz (for use with my ADS-B receiver).  As this was an exercise in a new construction technique, I used the DL6WU design as the basis for both.  This time I used John Drew’s Yagi Calculator to produce the dimensions (see the link at the bottom if you wish to download a copy of the software).  The resultant Yagi both appeared to work well.  The 1296 MHz 10 element was tested at home with ye olde Revex® W-570 VSWR meter and, without any adjustment to the elements, the Yagi gave a reading of 1.3:1.  The 1090 MHz 10-element Yagi was swept with the assistance of Alan, VK3XPD; with a little tweaking, a return loss of 23 dB was achieved.  This tweaking amounted to a little filing of the first director ends, and tilting the driven element forwards slightly.

 

Next I built a 28-element Yagi scaled for 1090 MHz. The 1090 MHz 28-element Yagi is currently in use at the top of my tower, and is a considerable improvement over the 16-element co-linear that I was using with my ADS-B receiver.  It has a calculated gain of 16.7 dBd.

 

With three successes under my belt, I felt it was time to start work on the new 1296 MHz array.  This would be four Yagi, each with 50 elements on 18 wavelength booms.  Work on this was a bit sporadic; fours hours spent cutting the 196 passive elements would leave me wanting to do almost anything else for a while...  But in a burst of work in January 2011 it all came together, and January 25th saw the array in place and operational at the top of my tower.

 

As a simple exercise in trying to improve signals on 1296 MHz, the new array is a marked improvement over the single Yagi it replaced.  AEP tests on 1296 MHz with Ian VK1BG, in the Canberra suburb of Hawker, have been taking place for a number of years now.  Using the new array, peak signals from Ian have improved 2 to 3 S-points.  Rather than having three or four "peaks" of 41 to 51 RS reports separated by nil signal troughs, we usually experience several minutes of 41-51 continuous signal, with two or three peaks of greater strength; 54 to 56, dependant on the size of the aircraft.

 

Before the array was completed, I wrote a construction article for "Amateur Radio" magazine.  It was published in the December 2009 edition of the magazine, and with the kind permission and assistance of the editor it is now also available here in PDF format.

 

VK3BJM T-Boom AR Dec-2009.pdf

More photos of the assembly of both the array and the ADSB receive Yagi are viewable here: http://www.flickr.com/photos/72319077@N00/sets/72157603461538031/

 

The VK5DJ Yagi Calculator is available here: http://www.vk5dj.com/yagi.html

 

 

 

 

 

Updated 18/5/2011

 

 

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T-BOOM YAGI